This invention relates to medical diagnostic x-ray apparatus and in particular to an x-ray apparatus for angiographic procedures.
X-ray equipment generally adapted to the needs of angiography is disclosed in U.S. Pat. No 4,358,856 issued on Nov.9, 1982 to P. Stivender et al. and shown schematically in FIG. 1(a). This equipment is comprised generally of opposing x-ray source 16 and an image receiver 14 mounted at either end of a "U" arm 12 and directed along an image axis 96 through an isocenter 80. An "L" arm 10 is pivoted on a first pivot 20 about a first machine axis 76, and supports a second pivot 18 on a second machine axis 78 perpendicular to the first machine axis 76. Rotation about the first and second machine axes permits imaging along an arbitrary image axis 96. Table 92, shown in phantom, is included to provide reference as to the position of a patient (not shown)
As shown in FIG. 1(b), a second approach used in the construction of angiographic x-ray equipment, and disclosed in U.S. Pat. No. 4,150,297 issued on Apr. 17, 1979 to Borggren comprises a "C" arm 22 in lieu of a U arm 12. In this approach the L arm 10 is pivotably mounted on a support beam 28 rather than on the floor to rotate about a first axis 76. The C arm 22 does not pivot about a second pivot on the L arm 10 but rather slides through a supporting collar 24 so as to cause the x-ray tube 16 and image receiver 14 located at either end of the C arm 22 to rotate about a second machine axis 78 perpendicular to the plane of the C arm 22. The radius of the C arm 22 is such as to position this second machine axis to intersect the isocenter 80. Therefore the resulting combined motion of the L arm and the C arm result in isocentric motion of the image receiver 14 and the x-ray source 16. The geometric analysis of a C arm system of FIG. 1(b) is similar to that of the U arm system of FIG. 1(b) after allowing for the 90.degree. offset of the first and second machine axis 76 and 78.
The above described L arm and C arm systems are not well adapted to certain radiographic procedures used to locate obstructions within the channel or lumen of a blood vessel. Referring to FIG. 2(a), plaque 42, such as that associated with arteriosclerotic disease, constricts the lumen 44 of coronary vessel 40. If the constriction is not concentric in cross section, as shown in FIG. 2(a), the constriction may not be apparent in a first projection of the vessel orientated perpendicularly to the vessel's major axis 50. Such a projection is shown in FIG. 2(c) as taken along projection angle 48 depicted in FIG. 2(a). At a different projection angle, however, such as angle 46 shown in FIG. 2(a), the constriction may be clearly visualized as shown in the projection of FIG. 2(b). Generally, in order to accurately assess the extent of the constriction of a vessel, the vessel must be "profiled" from several different angles. This requires that the image axis of the x-ray machine be rotated about the major axis of the vessel within a "profiling" plane perpendicular to the major axis of the vessel. In the case of the coronary arteries, this profiling plane may have a nearly arbitrary orientation corresponding to the many possible artery major axes. Each profiling plane may be identified by a "profiling plane axis" which is identical to the major axis of the vessel being profiled and perpendicular to the profiling plane.
Motion within an arbitrary profiling plane is difficult to achieve with the L arm and C arm systems described above. With such systems, profiling with a profiling plane axis which is not perpendicular to the first machine axis of the x-ray machine requires the simultaneous motion of both the L and U arm or L and C arm axes. Important, for profiling plane axes nearly parallel to the first machine axis, the L arm must swing through approximately the same arc as the arc that the image axis moves within the profiling plane. Such large L arm motion may be undesirable during a medical procedure to the extent that it interferes with equipment positioned near the operating table and disrupts the medical professionals attending the procedure.
A constant profiling motion, in the above described L and C systems, also requires that the relative speed and direction of the L and U arm or L and C arm axes be continually adjusted according to complex trigonometric relationships which are dependant on the relative location of each axis and the profiling plane axis. In most situations accurate profiling control of these systems is not possible under the direct control of a human operator.